Method of preparing and nitriding a ferrous article



A. W. MORRIS April 9, 1935.

METHOD OF PREPARING AND NITRIDING A FERROUS ARTICLE Original Filed Dec. 17, 1951 Patented Apr. 9, 1935 q UNITED STATES METHOD OF PREPARING AND NITRIDING A FERROUS ARTICLE Albert W." Morris, Springfield, Mass., assig'nor to Moore Drop Forg'ng Company, Springfield, Mass., a corporation of Massachusetts Original application December 17, 1931, Serial 581,701. Divided and this application August 25, 1934, Serial No. '741,512

2 Claims.

This invention relates to a new method of preparing and nitriding a ferrous article, the present case being a divisional application of my copending application Serial No. 581,'701, filed De- 6 cember 17, 1931.

Heretofore, so far as I am informed, in preparing a ferrous article for surface nitrogenization, allthe metal in the' article has been made of the same mixture or alloy. In other words, to get a surface suitable for nitridation, the entire piece has been made of a nitridable alloy or of the same composition throughout its mass.

This generally is a disadvantage, first because of the cost, and second because the elements included in the article to make it nitridable are usuany detrimental to the texture of the article and often interfere with or prevent heat treatment.

My improved method consists in impregnating the surface of the ferrous article to be treated to a suitable depth with a metal or alloy having aflinity for nitrogen. The article thus treated is then subjected to the action of nitrogen so that a hard nitrided surface will be obtained on the article and whereby the core or center of the article will be practically untreated and will retain the characteristics due to its original composition or alloy.

By my method almost any ferrous alloy having good properties of strength and that will be insensible to tempering brittleness can be used, thus insuring better castings, forgings and permitting heat treatment of ferrous articles having nitrided surfaces.

My method may be carried out as followsz- After making the ferrous article in the form desired, either by casting or forging I immerse or dip it, while hot, and preferably heated to a temperature about 1200 Fahrenheit, in a bath of molten metal having aiiinity for nitrogen. This bath is preferably molten aluminum either pure or alloyed with zirconium, titanium, chromium, vanadium, magnesium, beryllium, silicon, nickel, molydenum, manganese, or any desired combination of these, known in the art for alloying with iron to make up a nitridable alloy.

I have found by thus treating a ferrous article that there will be a surface impregnation thereof to a considerable depth by the metal or alloy 50 having afiinity for nitrogen.

I have also found that I can increase both the depth and amount of the impregnation of the ferrous article by subjecting the bath of molten metal having affinity for nitrogen to the action of agitation in any conventional manner, but preferably to agitation produced by the intermittent action of hydrostatic pressure.

The improved method may be carried out by many forms of apparatus, a preferred form being illustrated in the accompanying sheet of drawings forming part of this application fo patent. r

Referring to the drawing:

Fig. 1 is a sectional elevation, and

Fig. 2 is a ero& sectional view of the apparatus shown in Fig. 1

Referring more particularly tothe drawing in which like reference numerals designate like parts, A designates a heating chamber, and B a suitable crucible which can be placed in the chamber A, the crucible having lugs G which position the same in the heat-ing chamber. Gas pipes D-D extend through the walls of the chamber A and a mixture of air and gas may be led through these pipes, tangentially into the annular space between the heating chamber and crucible so that the metal in the crucible can be kept in molten condition and at a suitable temperature.

E designates a cover plate which is adapted to rest upon the top rim of the crucible B. This cover plate may be tightly clamped to the top of the crucible by screws F, which are pivoted in the lugs G on the crucible and which screws fit between ears C formed on the rim of the cover plate E, nuts H being threaded on the screws F.

A packing or hearing washer may be placed on top of the crucible. The cover plate E may be provided with a stufiing box I through which a plunger J can be reciprocated for purposes hereinafter described. This plunger may be made of graphite or may have its surface coated with graphite. Extendjng from the cover plate is a notched fiange K for holding the ferrous articles to be treated.

Practically any ferrous article can be treated in the apparatus above described. As shown a number of automobile shafts L are being treated. With the above described apparatus, the shafts L after being shaped either by forgir'g or casting are placed in the notches in the fiange K.

Care should be taken to keep the articles from touching each other, while in the bath, as such touching would hinder impregnation of the articles at the points of contact and would result in thin Spots in the coating.

These shafts or ferrous articles to be treated when placed in position should be very hot, preferably heated to a temperature above 1200 Fahrenheit. This temperature can be obtained by taking the articles while they are hot from forging or casting or they can be heated to the desired temperature before being placed in' the notches.

The crucible is filled with a bath of molten metal or alloy having affinity for nitrogen. The metal or alloy thus employed is preferably aluminum or any alloy that is known in the art as useful for affecting nitrogenization.

This bath is kept at the proper temperature by the gas jets D-D previously described.

The cover plate is then .placed in position on the crucible and tightly clamped thereto so that the hot ferrous articles will be immersed or dipped in the bath of molten metal. The articles are kept in the bath a suitable length of time.

During the treatment in the bath, care should be used to see that the surface of the ferrous articles to be impregnated are clean and free from scale. i V

I found that under these conditions, the metal or alloy having aflnity for nitrogen will penetrate to a sufiicient depth into the surface of the ferrous articles being treated to permit them to be nitrided after being machined to size.

This impregnation is accelerated by depressing the plunger J to create a hydrostatic pressure between the bath and the articles, and removing same, repeating the action intermittently, thereby to agitate the bath. Agitation may be obtained by reciprocating the 'plunger in contact with the bath, or for that matter in any conventional manner, as by rocking.

After the above treatment the cover plate is removed and the ferrous articles are allowed to cool. They then may be subjected to a finishing process such as grinding or machining to get the same to exact size.

Then the articles are subjected to the action of free or nascent nitrogen as in any ordinary nitrogenization or nitridation process.

Impregnation of surfaces not to be nitrided later may be prevented by protecting said surfaces by a suitable plating or paint. This also can be done after impregnation and before the time for nitridatlon.

For example, all of the surfaces of the crank shafts under treatment in the apparatus, except the hearing surfaces, may be protected so that the only surfaces which will be hardened or nitrided will be the bearings and the crank pins or journals.

The nitrogen will unite with the metal having affinity for nitrogen which has impregnated into the surface of the ferrous article.

Thus a ferrous article will be produced having any desired ferrous composition in its core and having a hard nitrided surface or surfaces. This article will thus have the advantages previously described, and 'is particularly adapted for heat treatment.

While I have shown a .preferred form of apparatus for practicing my improved method, other forms of apparatus may be devised whereby the surface impregnation, which is the principal feature of my invention, can be practiced, and I do not intend to be limited other than in the appended claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Pat.- ent of the United States is:-

1. The method of nitriding a ferrous article which consists of immersing the article while heated in a bath of molten metal having affinity for nitrogen and composed at least in part of aluminum, and agitating the molten metal by applying and removing a hydrostatic pressure on the bathv to obtain a surface impregnation of the ferrous article with said metal, removing the article from the bath and thereafter hardening the surface thereof by nitridation.

2. The method of treating a ferrous article, which consists in immersing the article while heated in a bath of molten metal having affinity for nitrogen and composed at least in part of aluminum, agitating the molten metal to obtain a surface impregnation of the ferrous article with said metal, removing the article from the bath and thereafter hardening the surface thereof by nitridation.

ALBERT W. MORRIS. 

